1. Field of the Invention
The present invention relates to a method and apparatus for forming through holes in a ceramic green sheet using a laser.
2. Description of the Related Art
Laminated electronic components are often manufactured by laminating a plurality of ceramic green sheets together. In order to permit electrical connection between the layers, through holes must be formed in the individual layers. Such through holes are conventionally formed by a punching process using a die and a pin. According to the conventional punching process, the dimensional precision of the die and the pin determines the precision of the through hole. However, this conventional process has several drawbacks.
When a through hole having a diameter of 100 micrometers or less must be formed, it is extremely difficult to achieve the desired precision. Also, since the die and the pin have a relatively short life and need to be periodically exchanged. Because the pin and die are expensive, this substantially increases the cost of using the conventional process. Another drawback of the conventional process is that different dies must be used for different types of electronic components. Since the exchange operation is complicated, this adds yet further to the cost of the conventional process.
As a result of these drawbacks, several methods for forming through holes with a laser beam have been proposed. Two such prior art methods, each of which enables accurate formation of through holes having a diameter of approximately 80 micrometers, use laser beams to form through holes in the ceramic green sheet. In accordance with these methods, a single shot of the laser forms a single through hole in the ceramic green sheet. To form a plurality of such through holes, both methods rely on relative movement between the laser beam and the ceramic green sheet. The relative movement ensures that the laser beam moves from point to point along the ceramic green sheet to cut the desired through holes. In the first of these methods, the ceramic green sheet is kept stationary while the laser beam is moved from point to point using a galvanomirror. In the second of these methods, the laser beam is kept stationary and the ceramic green sheet is moved. In both cases, each respective though hole is formed with a separate shot of the laser reducing the productivity of the method.
In both of these prior art methods, the diameter of the laser beam is equal to the desired diameter (approximately 80 micrometers) of the through holes making it possible to apply the laser beam directly to the ceramic green sheet without using a mask. However, this also creates potential problems because the shape of the through hole will be distorted if the laser beam is emitted obliquely.
To avoid these problems, a laser-beam punching method as shown in FIG. 8 has been proposed. According to this known laser-beam punching method, a mask 82 having a predetermined number of circular through hole transmissive portions 82a is disposed a significant distance from a ceramic green sheet 84 mounted on a table 87. A laser source 81 is used to emit a laser beam L, having a diameter larger than each through hole transmissive portion 82a, through a mask 82. As a result, the single laser beam L is divided into a plurality of sub beams L' forming a predetermined pattern of light on the ceramic green sheet 84 by means of a converging lens 83. As a result of this structure, a plurality of generally circular through holes are simultaneously formed in an irradiated area Lc on the ceramic green sheet 84.
While overcoming the problems of the above described laser punching methods, this method has its own drawbacks. Because the mask 82 is located a relatively large distance from the ceramic green sheet 84, there will be distortions in the shape of the through holes 86 formed along the perimeter of the irradiation area Lc. See FIG. 9. Only the though hole 85 (defined by the sub beam L' passing through the center opening of the mask 82) formed in the center of the irradiation area Lc will be circular in shape. The peripheral through holes (formed by the sub beams L' passing through the peripheral openings 82a in mask 82) will be elliptical in shape having inferior roundness and a large error from the desired shape. Accordingly, in order to form through holes which are of a desired roundness, it has been necessary to either form each through hole with a separate shot of the laser beam (the diameter of the beam being equal to the diameter of the through hole) or to use an expensive lens which will optically avoid such distortions. The former solution increases processing cost. The latter solution increases the cost of the processing equipment.